1 / 20

Mini Introduction to BGP

Mini Introduction to BGP. Michalis Faloutsos. What Is BGP?. Border Gateway Protocol BGP-4 The de-facto interdomain routing protocol BGP enables policy in routing: Which information gets advertised and how BGP is a Distance Vector like protocol

irvingm
Download Presentation

Mini Introduction to BGP

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Mini Introduction to BGP Michalis Faloutsos

  2. What Is BGP? • Border Gateway Protocol BGP-4 • The de-facto interdomain routing protocol • BGP enables policy in routing: • Which information gets advertised and how • BGP is a Distance Vector like protocol • Within an AS, Internal Gateway Protocol (IGP or I-BGP)

  3. How A BGP graph Looks Like AS 2 • Each AS has designated BGP routers • BGP routers of an AS communicate internally with another protocol (IGP) AS 5 AS 4 AS 3 AS 1

  4. What is different with BGP? • BGP goal: enable business relationships • Opts for: flexibility, scalability • Performance optimization is secondary

  5. Some Basic Numbers • ~20,000 Autonomous Systems approx. • Corporate Networks • ISP Internal Networks • National Service Providers • Identified by ASN a 16 bit value • Assigned by IANA • Superlinear growth

  6. IP Addresses and Prefixes • IP addresses have 32 bits: 4 octets of bits (IPv4) • A prefix is a group of IP addresses • 128.32.101.5 is an IP address (32 bits) • 128.32.0.0/16 is a prefix of the 16 first bits: • 128.32.0.0 – 128.32.255.255 (2^16 addresses) • 128.32.4.0/24 is a prefix of the 24 first bits - longer

  7. Routing is Based on Prefixes • A BGP Routing table has prefixes for entries • For a IP address of a packet, find longest match • Example: packet IP 128.32.101.1 • 128.32.0.0/16 match for 16 bits • 128.32.101.0/24 is a longer match • No matches: • 128.1.1.4 • 128.32.5.0/24

  8. Prefix Matching in More Detail • For a IP address of a packet, find longest match • Example: Compare • packet IP 128.32.101.1 • With 128.32.0.0/16 • IP : 01000000. 001000000. 01100101 .00000001 • Mask : 11111111. 111111111. 00000000 .00000000 • AND : 01000000. 001000000. 00000000 .00000000 • Prefix : 01000000. 001000000. 00000000. 00000000 • Equal? Yes

  9. Advertising Routing Information • Each AS advertises what it can reach from each BGP router • Policies I: filter what you advertise • Policies II: filter from what you hear advertised • Build up a BGP routing table • Remember which prefix you hear from which link

  10. What Does a Routing Table Look Like? • Origin AS “owns” the address • Routing tables can have peculiarities

  11. Route Advertising • Distance Vector style protocol • Hear advertisements: IP prefix, AS-path • Filter if desired (i.e. ignore) • Append yourself: IP prefix, myAS+AS-path • Forward to appropriate ASs

  12. Basic AS relationships • Customer – Provider • Customer pays Provider for service • The Customer is always right • Peer to Peer: mutual cooperation • Ex. MCI and AT&T • Sibling-Sibling • Ex. AT&T research and AT&T wireless

  13. Provider Customer Peer Peer The Internet as a Directed Graph • Every edge is bidirectional • Business relationships are represented

  14. The Initial Idea • Data flows between customers-providers • Top level providers are peers • They exchange information to ensure connectivity • What can possibly go wrong?

  15. And then came the rain… • Thousands of ASs • Complicated relationships • Multiple providers for one AS!! • Multihoming • Traffic engineering • I want to use multiple paths and load balance

  16. AS Relationships Provider Customer 200 100 • Customer – Provider: customer pays and is always right • Peer to Peer: Exchange traffic only between their customers • Sibling-Sibling: Exchange traffic at will Peer Peer 10 11 12 13 1 4 3 2

  17. The Rules of BGP Routing • Transit traffic: traffic that does not go to my customers (or their customers) • A provider carries any traffic to or from a customer • Peers exchange traffic only if between their customers

  18. Routing rules: Provider accept everything Peer only if it is for its customers Path Properties: Up then down No up-down-up, at most 1 peer-peer steps How BGP Policy Restricts Routing Provider Customer Peer 100 Peer 200 10 11 13 12 1 3 4 2

  19. Implementing BGP Rules • What do you do with an advertisement: • Through customer link • Advertise to all (customers, peers, providers) • Through provider link • Advertise to customer only (and possibly siblings) • Through peer link • Advertise to customer only (and possibly siblings) • Through sibling link • Advertise to all

  20. How Policies Affect Routing • A Provider will get rid of traffic as soon as possible, • But a Provider will carry the traffic for its customer • Did anyone say traffic is asymmetric? Customer 1 ISP1 ISP2 Customer 2

More Related